Pump-probe X-ray reflectivity

A research team led by Professor J. Larsson (Lund University, Sweden) has recently performed time-resolved X-ray reflectivity measurements with 100 picosecond resolution at ID09B, at the European Synchrotron Radiation Facility (ESRF). The experiment is a so-called pump-probe measurement, i.e., the repetition of the measurement with systematic change of the delay time of the pump (laser light) and probe (X-ray) pulses. In their research, amorphous carbon films with a thickness of 46 nm were excited with laser pulses (100 fs duration, 800 nm wavelength, and 70 mJ/cm² fluence). Here, the laser-induced stress caused a rapid expansion of the thin film followed by a relaxation of the film thickness as heat diffused into the silicon substrate. The researchers succeeded in measuring changes in film thickness by X-ray reflectivity with a short X-ray pulse (100 ps duration). It was observed that thermal stress generated by laser excitation causes the film to rapidly expand and increases the surface roughness substantially. The subsequent relaxation of film thickness is governed by heat diffusion into the substrate. For more information, see the paper, "Picosecond time-resolved x-ray reflectivity of a laser-heated amorphous carbon film", R. Nuske et al., Appl. Phys. Lett. 98, 101909 (2011).